The rapid globalization of world economy, expansion of world population and rise of living standards lead to tremendous energy demand. Since the present oil and gas supply could hardly always match the rapidly rising market demand, their prices are escalating and unpredictable. In particular, due to the difficulties of storage and overseas transportation, the natural gas price is more volatile.
The separation of selected hydrocarbon components from a water-saturated hydrocarbon gas mixture (abbreviated as “raw gas” hereafter), in particular, the separation of the natural gas liquid (abbreviated as “NGL” hereafter) from natural gas, would consume a significant portion of the raw gas. The gas is consumed not only as the raw material, but also as the fuel to generate heat and power required for the energy-intensive separation process. The production cost of NGL, therefore, escalates with the gas price. However, the NGL is not the end-use product itself. As a liquid mixture, the NGL product must be further fractionated into its components and sold to different end users on the market. The market price of each NGL component would fluctuate according to their respective market demands. As a consequence, their respective net profit would be quite different. For instance, the market price of ethane rose much slower recently than other components when gas price went up. The net profit of ethane production may even become negative at peak gas price. On the contrary, the net profits of certain other NGL component, such as propane, would become more attractive due to the increasing LPG demand.
In such a volatile NGL components market, the prevailing Cryogenic Process for NGL separation is facing a serious challenge. The energy-intensive Cryogenic process has been greatly favored in the past because of its high ethane recovery level (˜90%). Facing the shrinking profit of ethane production, however, the overall net profits of existing Cryogenic separation plants have been declining. Many Cryogenic separation plants had to operate on so-called “ethane-rejection” cycle. In the ethane-rejection operations, the liquid ethane, separated after gas expansion, had to be re-evaporated from the NGL mixture and re-compressed to the original gas pressure, mixed with the residue gas, and sold at the same price of the raw gas. The energy consumed in the whole ethane-separation and ethane-rejection processes was completely wasted. The overall profit of the Cryogenic plants, therefore, would be decreasing faster due to its higher ethane recovery rate as compared with other separation processes. As a result, the future of the Cryogenic process becomes uncertain.
Numerous improvements have been proposed for lowering ethane-rejection costs. Among so-called “Next Generation” Cryogenic processes, a number of US patents have been granted, such as the U.S. Pat. Nos. 4,854,955; 4,889,545; 5,568,737; 5,711,712; 5,799,507 and 5,881,569. Since all these patents are still based on cryogenic expansion cycle wherein the bulk raw gas must be expanded to sufficiently low pressure to create the required low cryogenic temperature, the residue gas has to be recompressed to pipeline pressure before sent out. The recompression power, therefore, could not be significantly reduced. On the other hand, the recovered NGL decreases substantially due to the rejection of ethane. As a result, the average energy cost per unit of the NGL products would rapidly increase, and the overall plant net profit drops accordingly. All these improved “Next Generation” cryogenic processes, therefore, could hardly make Cryogenic process more competitive in future NGL market.
A different approach based on the Absorption process has also been proposed to solve the problem. The traditional refrigerated oil absorption process (abbreviated as “ROA process” hereafter) failed to compete with Cryogenic process due to its lower ethane recovery rate and higher refrigeration energy consumption. Presently, the first weakness is no longer a drawback. To eliminate the second weakness, i.e., higher refrigeration energy consumption, a lot of efforts have been made to find alternative absorbents that could selectively absorb heavier hydrocarbons at ambient temperature. For example, Dr. Mehra has identified a number of such novel absorbents and received a series of U.S. Pat. Nos. 4,421,535; 4,511,381; 4,526,594; 4,578,094; 4,698,688; 5,561,988; and 5,687,584, etc. He has also made notable progresses in commercializing his process during past two decades. However, due to the higher costs and the lack of industrial operational experience of those novel absorbents, the market share of the so-called “Mehra process” is still limited, and does not have significant impact on gas industry. It is, therefore, not expected to become a widely accepted alternative to the declining Cryogenic process in foreseeable future.
Another approach based on radically improvements on traditional ROA process has been proposed more recently by the present inventor to substantially reduce the refrigeration energy consumption while still using similar heavy oil absorbents under refrigeration temperature. The basic idea is to integrate the technology of refrigeration dehydration of natural gas with the improved heat-recuperated rich oil processing process. In April 2003, the U.S. Pat. No. 6,553,784 “Comprehensive Natural Gas Processor” based on an innovative “Improved Refrigeration Oil Absorption Process” (abbreviated as “IROA process” hereafter) was granted. The IROA process could substantially reduce the required refrigeration power and heat energy and effectively meet the challenge of the volatile NGL market, while still inheriting the matured experience of traditional ROA process. According to a preliminary computer simulation of the IROA process, as reported on AIChE 2005 Spring Meeting, April 11-14 2005, Atlanta, the IROA process could compete favorably with Cryogenic process over a wide range of ethane recovery rates, notably from ˜70% down to ˜2%, for a wide variety of raw gas. The widely accepted expertise of the traditional ROA process currently in operation would give excellent market penetration prospect of the IROA process under development.
However, being emphasized on the improvements on the head and bottom ends of the ROA process, said patent has not fully explored the potential of the flexibility and energy-savings of the IROA process over the entire ethane recovery range between >95% down to <2%, particularly at the highest and the lowest ends of the ethane recovery range, in comparison with the Mehra process.
Accordingly, it is an objective of the present invention to provide a fully flexible improved refrigerated absorption process (abbreviated as “FIRA process” hereafter) that could control the desirable ethane recovery rate from >95% down to <2% during operations to best meet the challenge of the volatile NGL market.
Another objective of the present invention is to provide a fully energy-integrated FIRA apparatus that could substantially reduce the unit product energy consumption and maximize the profits of the FIRA gas separation plant during operations over the full range of ethane recovery rate from >95% down to <2%.
A further objective is to provide high-performance components for the FIRA process and reduce the capital costs of the FIRA gas separation plant.